1. Field of the Invention
The present invention relates to a pattern inspection apparatus and method, and more particularly to an apparatus and a method for inspecting a fine pattern, such as a semiconductor integrated circuit (LSI), a liquid crystal panel, and a photomask (reticle) for the semiconductor or the liquid crystal panel, which are fabricated based on data for fabricating the fine pattern such as design data.
2. Description of the Related Art
For the pattern inspection of a wafer in a fabricating process of semiconductor integrated circuit or the pattern inspection of a photomask for pattern formation of a wafer, an optical pattern inspection apparatus that uses the die-to-die comparison method has been used. In the die-to-die comparison method, a defect is detected by comparing an image obtained from a die to-be-inspected and an image obtained from the equivalent position of a die adjacent to the die to-be-inspected. In this case, the die means a semiconductor device.
On the other hand, for the inspection of a photomask (reticle) having no adjacent die, the die-to-database comparison method has been used. In the die-to-database comparison method, mask data is converted into an image. Then the image is used for a substitution of the image of the adjacent die used in the die-to-die comparison method, and inspection is performed in the same manner as the above. Here, the mask data is data obtained by applying photomask correction to design data. The technology concerned is disclosed, for example, in U.S. Pat. No. 5,563,702, “Automated photomask inspection apparatus and method.”
However, by using the die-to-database comparison method for wafer inspection, corner roundness of a pattern formed on a wafer is likely to be detected as a defect. In the inspection of a photomask, a pretreatment, which adds corner roundness to the image converted from the mask data by applying a smoothing filter, prevents the corner roundness of the pattern from being detected as the defect. However, in the inspection of a wafer, because the corner roundness added by the pretreatment may be different from corner roundness of each pattern actually formed on the wafer, the pretreatment may not perfectly prevent the corner roundness of the pattern from being detected as the defect. Therefore, an allowable pattern deformation quantity should be set in order to ignore the above difference. As a result, a problem in which a fine defect existing in a place except a corner cannot be detected has happened.
The above problem is not severe for the die-to-database comparison photomask inspection, because the photomask should correspond to the mask data as much as possible. Therefore, currently, the die-to-database comparison photomask inspection has been put into practical use. However, the above problem is severe for the die-to-database comparison wafer inspection, because a pattern formed on the wafer is allowed to be deformed as long as an electrical property is guaranteed. This allowable pattern deformation quantity is considerably large. Actually, pattern deformation occurs due to a difference in stepper exposure conditions, or the like. Therefore, the die-to-database comparison wafer inspection has not been put into practical use.
From a viewpoint of problems in semiconductor integrated circuit fabrication, repeated defects (systematic defects) are more important issue than a random defect caused by a particle or the like. The repeated defects are defined as defects that occur repeatedly over all dies on a wafer caused by photomask failure, or the like. Because the repeated defects occur in a die to-be-inspected and in adjacent dies that are to be compared with the die to-be-inspected, the die-to-die comparison wafer inspection cannot detect the repeated defects. Therefore, the die-to-database comparison wafer inspection has been demanded.
Although the die-to-die comparison wafer inspection has not been put into practical use because of calculation cost or the like, there have been proposed inspection methods in which design data and a wafer image are used. These inspection methods are disclosed in, for example, a literature: “Automatic failure part tracing method of a logic LSI using an electron beam tester,” NEC Technical Report, vol. 50, No. 6, 1997. In this literature, the following methods are disclosed: a method in which projections of wiring edges on the X- and Y-axes are used; a method in which wiring corners are focused on; and a method in which a genetic algorithm is applied. Moreover, as a method used in this literature, a matching method in which after edges are approximated by straight lines, closed areas are extracted, and the closed areas are used for matching is described. However, these methods fail to realize an inspection speed that is usable in high-speed inspection, and fail to perform matching while detecting a pattern deformation quantity.
Further, currently, the automatic defect classification (ADC) method in which an image of a die having a defect is used has been used. However, the method cannot classify whether the defect is a killer defect or not, because the method cannot recognize which part of a circuit the defect destroys.
Moreover, a position of a defect detected by the die-to-die comparison inspection has an error caused by precision of a stage and an image generation device of an inspection apparatus, and such error is approximately ten or more times larger than a line width of a pattern to-be-inspected. Due to the error, even if a defect position is related with design data, relationship between the defect position and the design data cannot be recognized.
Currently, a line width of semiconductor integrated circuits is far shorter than wavelength used in a lithography process. In the lithography process, a method of adding an OPC (Optical Proximity Correction) pattern has been used. In the method, by using a photomask fabricated by mask data that is generated by adding an OPC pattern to design data, a pattern formed on a wafer fabricated by the photomask can be consistent with the design data as much as possible. Adding an OPC pattern is one of the most important techniques for photomask correction.
If the OPC pattern does not effectively correct a pattern formed on a wafer, repeated defects occur. However, the die-to-die comparison wafer inspection cannot detect the repeated defects. In order to solve this problem, it is necessary to provide a method in which a pattern formed on the wafer is inspected based on design data in consideration of an allowable pattern deformation quantity.
In addition, in a multi-product/small-volume fabricating process, e.g. a system-on-a-chip (SoC) fabricating process, a short delivery time is required. In the fabricating process, when repeated defects are detected in electric inspection as a final inspection, a short delivery time cannot be achieved. In order to solve this problem, it is necessary to provide an inspection method that inspects a difference between a pattern formed on a wafer and design data for each lithography process. In the inspection method, it is necessary for an allowable pattern deformation quantity that does not affect an electrical property to be set, and a deformation quantity that exceeds the allowable pattern deformation quantity should be detected.
Further, a lithography simulator inspects a simulation pattern, which is obtained from mask data to which an OPC pattern is added, by comparing the simulation pattern with design data in order to evaluate the OPC pattern. Although the entire device can be verified by the lithography simulator, the simulation pattern cannot be necessarily the same as an actual pattern. Moreover, a defect except for a defect caused by the OPC pattern cannot be detected by the lithography simulator. A random defect existing on a photomask, a stepper aberration, or the like is an example of such defect.
Moreover, for verifying the simulation, it is necessary to provide a method in which a simulation pattern outputted from the lithography simulator is verified with an image of a pattern actually formed on a wafer. Moreover, it becomes increasingly important to improve the technology for circuit design by setting an allowable pattern deformation quantity to design data precisely and in detail.
A CD-SEM (Critical Dimension Scanning Electron Microscope) has been used for controlling a line width in a fabricating process of semiconductor integrated circuits. The CD-SEM automatically measures a line width of a line-shaped pattern in a specified position using a profile (line profile). Several positions in several shots on several pieces of wafers for each lot are measured in order to control stepper exposure condition by using the CD-SEM.
As control items in a fabricating process of semiconductor integrated circuits, end shrinkage of a wiring, a position of an isolated pattern, and the like are also important besides the line width, but the automatic measuring function of the CD-SEM allows only one-dimensional measurement. Specifically, the CD-SEM can measure only a length such as a line width. Therefore, those two-dimensional shapes are inspected by an operator manually using an image acquired from the CD-SEM or other microscopes.
The isolated pattern includes a hole pattern or an island pattern. The island pattern may be a negative pattern of the hole pattern. The hole pattern includes a contact hole/via hole or a test pattern.
Generally, an OPC pattern plays an important role not only to guarantee a gate line width, but also to form shapes of a corner and an isolated pattern. Furthermore, because of improvement of a processor frequency, control of a shape of an end of a gate pattern, which is called an end-cap, or a base of a gate pattern, which is called a field extension, also becomes important in addition to the gate line width.
The above inspections of two-dimensional patterns are essential both in sampling inspection in a fabricating process, and in an R&D fabricating process. Especially, in the R&D fabricating process, it is necessary to inspect all patterns formed on a wafer. However, currently, the control of the two-dimensional shape is performed by a human work, and is not perfectly performed. In order to solve this problem, automated die-to-database comparison wafer inspection is required.
As concrete subjects for automatization, the following subjects are enumerated:
1. The method, in which the inspection is performed for every profile acquisition section, is used. However, by using such local inspection method, deformation for every pattern to-be-inspected cannot be inspected.
2. It is necessary to detect bridge state of a pattern to-be-inspected, which is a negative pattern in a wiring layer, in higher speed. Further, it is necessary to reduce an effect of an electron beam spot size.
3. In the case where a pattern to-be-inspected, which is a negative pattern in a wiring layer and has a larger line width than an electron beam spot size, does not exist, it is necessary to prevent deterioration of defect detection accuracy of a pattern to-be-inspected, which is a negative pattern in a wiring layer and has the smaller line width than the electron beam spot size.
4. Conventionally, an overlay error caused in process of the double exposure method is controlled by measuring limited areas in a semiconductor device. Therefore, the overlay error caused locally by a stepper aberration or the like cannot be controlled.
5. In the case of long-term inspection, it may be necessary to adjust a setting value of an image generation device. In such case, it is necessary that the most suitable setting value of the image generation device is estimated and the setting value of the image generation device is adjusted by using the estimated setting value.
6. A method of controlling OPC patterns of gates by using a line width and space widths on both sides of gates has been used. Because space widths on both sides of a gate having different space widths on one side cannot be determined uniquely, OPC patterns of the gate cannot be controlled.
7. A part of a pattern to-be-inspected fabricated by a stepper under a focus condition and an exposure dose condition, which are different from the optimal conditions, may be deformed more than an allowable deformation quantity. In such case, pattern deformation quantities cannot be obtained from the above-mentioned part.